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When using WAAS at an alternate airport, flight planning must be based
on flying the RNAV (GPS) LNAV or

circling minima line, or minima on a GPS approach procedure, or
conventional approach procedure with "or

GPS" in the title. Code of Federal Regulation (CFR) Part 91 non

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precision weather requirements must be used

for planning. Upon arrival at an alternate, when the WAAS navigation
system indicates that LNAV/VNAV or

LPV service is available, then vertical guidance may be used to
complete the approach using the displayed level
of service. The FAA has begun removing the

NA

 (Alternate Minimums Not Authorized) symbol from select

RNAV (GPS) and GPS approach procedures so they may be used by approach
approved WAAS receivers at
alternate airports. Some approach procedures will still require the

NA

 for other reasons, such as no weather

reporting, so it cannot be removed from all procedures. Since every
procedure must be individually evaluated,
removal of the

NA

 from RNAV (GPS) and GPS procedures will take some time.

NOTE

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Properly trained and approved, as required, TSO-C145() and TSO-C146()
equipped users (WAAS users) with and using

approved baro-VNAV equipment  may plan for LNAV/VNAV DA at an alternate
airport. Specifically authorized WAAS users

with and using approved baro-VNAV equipment may also plan for RNP 0.3
DA at the alternate airport as long as the pilot

has verified RNP availability through an approved prediction program.

d.  Flying Procedures with WAAS

WAAS receivers support all basic GPS approach functions and provide
additional capabilities. One of

the major improvements is the ability to generate glide path guidance,
independent of ground equipment or

barometric aiding. This eliminates several problems such as hot and
cold temperature effects, incorrect altimeter

setting, or lack of a local altimeter source. It also allows approach
procedures to be built without the cost of

installing ground stations at each airport or runway. Some approach
certified receivers may only generate a glide

path with performance similar to Baro

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VNAV and are only approved to fly the LNAV/VNAV line of minima

on the RNAV (GPS) approach charts. Receivers with additional capability
(including faster update rates and

smaller integrity limits) are approved to fly the LPV line of minima.
The lateral integrity changes dramatically

from the 0.3 NM (556 meter) limit for GPS, LNAV, and LNAV/VNAV approach
mode, to 40 meters for LPV.

It also provides vertical integrity monitoring, which bounds the
vertical error to 50 meters for LNAV/VNAV and

LPVs with minima of 250' or above, and bounds the vertical error to 35
meters for LPVs with minima below

250'.

When an approach procedure is selected and active, the receiver will
notify the pilot of the most accurate

level of service supported by the combination of the WAAS signal, the
receiver, and the selected approach, using

the naming conventions on the minima lines of the selected approach
procedure. For example, if an approach

is published with LPV minimnd the receiver is only certified for
LNAV/VNAV, the equipment would indicate

"LNAV/VNAV available," even though the WAAS signal would support LPV. I
f flying an existing

LNAV/VNAV procedure with no LPV minima, the receiver will notify the
pilot "LNAV/VNAV available," even

if the receiver is certified for LPV and the signal supports LPV. If
the signal does not support vertical guidance

on procedures with LPV and/or LNAV/VNAV minima, the receiver
annunciation will read "LNAV available."

On lateral only procedures with LP and LNAV minima the receiver will
indicate "LP available" or "LNAV

available" based on the level of lateral service available. Once the
level of service notification has been given,

the receiver will operate in this mode for the duration of the approach
procedure, unless that level of service

becomes unavailable. The receiver cannot change back to a more accurate
level of service until the next time an

approach is activated.

NOTE

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Receivers do not "fail down" to lower levels of service once the
approach has been activated. If only the vertical off flag

appears, the pilot may elect to use the LNAV minima if the rules under
which the flight is operating allow changing the type

of approach being flown after commencing the procedure. If the lateral
integrity limit is exceeded on an LP approach, a

missed approach will be necessary since there is no way to reset the
lateral alarm limit while the approach is active.

Another additional feature of WAAS receivers is the ability to exclude
a bad GPS signal and continue

operating normally. This is normally accomplished by the WAAS
correction information. Outside WAAS

coverage or when WAAS is not available, it is accomplished through a
receiver algorithm called FDE. In most

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Navigation Aids